Medical photometer and medical photometry system

ABSTRACT

A medical photometer includes a signal producing section that produces a first control signal to emit a first light having a first wavelength, a second control signal to emit a second light having a second wavelength, a third control signal to emit a third light having a third wavelength, and a fourth control signal to emit a fourth light having a fourth wavelength, a signal acquiring section that acquires a first to fourth intensity signals, a processor, and a memory that stores instructions. In the medical photometer, the first wavelength and the second wavelength are selected as two wavelengths at each of which an extinction coefficient of blood is a first value. The third wavelength and the fourth wavelength are selected as two wavelengths at each of which the extinction coefficient of the blood is a second value which is different from the first value.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. application Ser.No. 16/134,539, filed Sep. 18, 2018, which is based on Japanese PatentApplications No. 2017-180016 filed on Sep. 20, 2017, the contents ofwhich are incorporated herein by reference.

BACKGROUND

The presently disclosed subject matter relates to a medical photometer,and a medical photometry system including the medical photometer.

Japanese Patent No. 4,196,209 discloses a pulse photometer which is anexample of a medical photometer. A pulse photometer is a device forcalculating the arterial oxygen saturation of a subject as an example ofthe photometry. Specifically, the tissue of the subject is irradiatedwith light beams respectively having a plurality of wavelengths at whichratios of the extinction coefficients of the blood are different fromeach other. The quantities of the light beams of the respectivewavelengths which are transmitted through or reflected from aredetected. The quantities of the light beams of the respectivewavelengths are changed in accordance with the pulsation of the blood ofthe subject.

Therefore, temporal changes of the quantities of the light beams of therespective wavelengths are acquired as pulse wave signals. The amplitudeof the pulse wave signal relating to each of the wavelengths correspondsto the light attenuation variation at the wavelength. The arterialoxygen saturation is calculated based on a ratio of the lightattenuation variations at the wavelengths.

Recently, it has been known that excessive administration of oxygen to asubject imposes a burden on the subject. In accordance withadministration of oxygen, the arterial oxygen saturation in the blood ofthe subject is increased. Namely, the arterial oxygen saturation can beused as an index of knowing the level of administration of oxygen to thesubject. When the arterial oxygen saturation reaches 100%, however, itis difficult to determine whether administration of oxygen is excessive.

A novel index which, when the arterial oxygen saturation of a subject is100%, enables to determine whether the amount of administration ofoxygen to the subject is adequate is provided.

SUMMARY

According to an aspect of the presently disclosed subject matter, amedical photometer includes:

a signal producing section that produces:

-   -   a first control signal that causes a first light emitter to emit        a first light beam having a first wavelength;    -   a second control signal that causes a second light emitter to        emit a second light beam having a second wavelength;    -   a third control signal that causes a third light emitter to emit        a third light beam having a third wavelength; and    -   a fourth control signal that causes a fourth light emitter to        emit a fourth light beam having a fourth wavelength;

a signal acquiring section that acquires:

-   -   a first intensity signal corresponding to an intensity of the        first light beam which is transmitted through or reflected from        tissue of a subject;    -   a second intensity signal corresponding to an intensity of the        second light beam which is transmitted through or reflected from        the tissue;    -   a third intensity signal corresponding to an intensity of the        third light beam which is transmitted through or reflected from        the tissue; and    -   a fourth intensity signal corresponding to an intensity of the        fourth light beam which is transmitted through or reflected from        the tissue;

a processor; and

a memory that stores instructions executable by the processor,

wherein the first wavelength and the second wavelength are selected astwo wavelengths at each of which an extinction coefficient of blood in astate where an oxygen saturation is 100% is a first value,

the third wavelength and the fourth wavelength are selected as twowavelengths at each of which the extinction coefficient of the blood ina state where the oxygen saturation is 100% has a second value which isdifferent from the first value, and,

when the instructions are executed by the processor, a venous oxygensaturation in a state where an arterial oxygen saturation of the subjectis 100% is calculated based on the first control signal, the secondcontrol signal, the third control signal, the fourth control signal, thefirst intensity signal, the second intensity signal, the third intensitysignal, and the fourth intensity signal.

According to another aspect of the presently disclosed subject matter, amedical photometry system includes:

a first light emitter that emits a first light beam having a firstwavelength;

a second light emitter that emits a second light beam having a secondwavelength;

a third light emitter that emits a third light beam having a thirdwavelength;

a fourth light emitter that emits a fourth light beam having a fourthwavelength;

a light detector that outputs a first intensity signal corresponding toan intensity of the first light beam that is transmitted through orreflected from tissue of a subject, a second intensity signalcorresponding to an intensity of the second light beam that istransmitted through or reflected from the tissue, a third intensitysignal corresponding to an intensity of the third light beam that istransmitted through or reflected from the tissue, and a fourth intensitysignal corresponding to an intensity of the fourth light beam that istransmitted through or reflected from the tissue; and

a medical photometer to which the first light emitter, the second lightemitter, the third light emitter, the fourth light emitter, and thelight detector are connected with wire or wireless connection,

wherein the medical photometer includes:

-   -   a signal producing section that produces a first control signal        that causes the first light emitter to emit the first light        beam, a second control signal that causes the second light        emitter to emit the second light beam, a third control signal        that causes the third light emitter to emit the third light        beam, and a fourth control signal that causes the fourth light        emitter to emit the fourth light beam;    -   a signal acquiring section that acquires the first intensity        signal, the second intensity signal, the third intensity signal,        and the fourth intensity signal;    -   a processor; and    -   a memory that stores instructions executable by the processor,

the first wavelength and the second wavelength are selected as twowavelengths at each of which an extinction coefficient of blood in astate where an oxygen saturation is 100% has a first value,

the third wavelength and the fourth wavelength are selected as twowavelengths at each of which the extinction coefficient of the blood ina state where the oxygen saturation is 100% has a second value that isdifferent from the first value, and,

when the instructions are executed by the processor, a venous oxygensaturation in a state where an arterial oxygen saturation of the subjectis 100% is calculated based on the first control signal, the secondcontrol signal, the third control signal, the fourth control signal, thefirst intensity signal, the second intensity signal, the third intensitysignal, and the fourth intensity signal.

According to another aspect of the presently disclosed subject matter, amedical photometer includes:

a signal acquiring section that acquires:

-   -   a first intensity signal corresponding to an intensity of a        first light beam which is transmitted through or reflected from        tissue of a subject, and which has a first wavelength;    -   a second intensity signal corresponding to an intensity of a        second light beam which is transmitted through or reflected from        the tissue, and which has a second wavelength;    -   a third intensity signal corresponding to an intensity of a        third light beam which is transmitted through or reflected from        the tissue, and which has a third wavelength; and    -   a fourth intensity signal corresponding to an intensity of a        fourth light beam which is transmitted through or reflected from        the tissue, and which has a fourth wavelength;

a processor; and

a memory that stores instructions executable by the processor,

wherein the first wavelength and the second wavelength are selected astwo wavelengths at each of which an extinction coefficient of blood in astate where an oxygen saturation is 100% has a first value,

the third wavelength and the fourth wavelength are selected as twowavelengths at each of which the extinction coefficient of the blood ina state where the oxygen saturation is 100% has a second value that isdifferent from the first value, and,

when the instructions are executed by the processor, the medicalphotometer

-   -   acquires a first signal set including the first intensity        signal, second intensity signal, third intensity signal, and        fourth intensity signal in a state where the tissue is not        pressurized,    -   acquires a second signal set including the first intensity        signal, second intensity signal, third intensity signal, and        fourth intensity signal in a state where the tissue is        pressurized, and    -   calculates a venous oxygen saturation in a state where an        arterial oxygen saturation of the subject is 100%, based on the        first signal set and the second signal set.

According to another aspect of the presently disclosed subject matter, amedical photometry system includes:

a first light emitter that emits a first light beam having a firstwavelength;

a second light emitter that emits a second light beam having a secondwavelength;

a third light emitter that emits a third light beam having a thirdwavelength;

a fourth light emitter that emits a fourth light beam having a fourthwavelength;

a light detector that outputs a first intensity signal corresponding toan intensity of the first light beam that is transmitted through orreflected from tissue of a subject, a second intensity signalcorresponding to an intensity of the second light beam that istransmitted through or reflected from the tissue, a third intensitysignal corresponding to an intensity of the third light beam that istransmitted through or reflected from the tissue, and a fourth intensitysignal corresponding to an intensity of the fourth light beam that istransmitted through or reflected from the tissue; and

a medical photometer to which the first light emitter, the second lightemitter, the third light emitter, the fourth light emitter, and thelight detector are connected with wire or wireless connection,

wherein the medical photometer includes:

-   -   a signal acquiring section that acquires the first intensity        signal, the second intensity signal, the third intensity signal,        and the fourth intensity signal;    -   a processor; and    -   a memory that stores instructions executable by the processor,

the first wavelength and the second wavelength are selected as twowavelengths at each of which an extinction coefficient of blood in astate where an oxygen saturation is 100% has a first value,

the third wavelength and the fourth wavelength are selected as twowavelengths at each of which the extinction coefficient of the blood ina state where the oxygen saturation is 100% has a second value that isdifferent from the first value, and,

when the instructions are executed by the processor, the medicalphotometer

-   -   acquires a first signal set including the first intensity        signal, second intensity signal, third intensity signal, and        fourth intensity signal in a state where the tissue is not        pressurized,    -   acquires a second signal set including the first intensity        signal, second intensity signal, third intensity signal, and        fourth intensity signal in a state where the tissue is        pressurized, and    -   calculates a venous oxygen saturation in a state where an        arterial oxygen saturation of the subject is 100%, based on the        first signal set and the second signal set.

According to the above-described modes, even when the arterial oxygensaturation of a subject is 100%, it is possible to determine whether theamount of administration of oxygen to the subject is adequate, based onthe calculated venous oxygen saturation. In other words, a novel indexis provided which, when the arterial oxygen saturation of a subject is100%, enables to determine whether the amount of administration ofoxygen to the subject is adequate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a functional diagram illustrating a functional configurationof a medical photometry system of an embodiment.

FIG. 2 is a graph for explaining a principle of calculating a venousoxygen saturation.

FIG. 3 is a flowchart illustrating an operation example of a medicalphotometer in FIG. 1 .

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment will be described in detail with reference tothe accompanying drawings. FIG. 1 illustrates the functionalconfiguration of a medical photometry system 1 of the embodiment.

The medical photometry system 1 includes a first light emitter 11, asecond light emitter 12, a third light emitter 13, a fourth lightemitter 14, a light detector 20, and a medical photometer 30.

The first light emitter 11 is configured so as to emit a first lightbeam having a first wavelength Xi. An example of the first wavelength Xiis 660 nm. The first light emitter 11 is a semiconductor light emittingelement which emits the first light beam. Examples of the semiconductorlight emitting element are a light emitting diode (LED), a laser diode,and an organic electroluminescence (EL) element.

The second light emitter 12 is configured so as to emit a second lightbeam having a second wavelength λ₂. An example of the second wavelengthλ₂ is 700 nm. The second light emitter 12 is a semiconductor lightemitting element which emits the second light beam. Examples of thesemiconductor light emitting element are a light emitting diode (LED), alaser diode, and an organic electroluminescence (EL) element.

The third light emitter 13 is configured so as to emit a third lightbeam having a third wavelength λ₃. An example of the third wavelength λ₃is 645 nm. The third light emitter 13 is a semiconductor light emittingelement which emits the third light beam. Examples of the semiconductorlight emitting element are a light emitting diode (LED), a laser diode,and an organic electroluminescence (EL) element.

The fourth light emitter 14 is configured so as to emit a fourth lightbeam having a fourth wavelength λ₄. An example of the fourth wavelengthλ₄ is 730 nm. The fourth light emitter 14 is a semiconductor lightemitting element which emits the fourth light beam. Examples of thesemiconductor light emitting element are a light emitting diode (LED), alaser diode, and an organic electroluminescence (EL) element.

The light detector 20 is an optical sensor having sensitivities to thefirst wavelength λ₁, the second wavelength λ₂, the third wavelength λ₃,and the fourth wavelength λ₄. The light detector may be configured so asto further have sensitivities to other wavelengths. Examples of theoptical sensor are a photodiode, a phototransistor, and a photoresistor.

The light detector 20 is configured so as to output a first intensitysignal S_(o1) corresponding to the intensity I₁ of the first light beamwhich is transmitted through or reflected from tissue of a subject. Thefingertip or earlobe of the subject may be used as an example of thetissue. The light detector 20 is configured so as to output a secondintensity signal S_(o2) corresponding to the intensity I₂ of the secondlight beam which is transmitted through or reflected from the tissue.The light detector 20 is configured so as to output a third intensitysignal S_(o3) corresponding to the intensity I₃ of the third light beamwhich is transmitted through or reflected from the tissue. The lightdetector 20 is configured so as to output a fourth intensity signalS_(o4) corresponding to the intensity I₄ of the fourth light beam whichis transmitted through or reflected from the tissue.

The medical photometer 30 has wired or wireless connections to the firstlight emitter 11, the second light emitter 12, the third light emitter13, the fourth light emitter 14, and the light detector 20. The medicalphotometer 30 includes a signal producing section 31, a signal acquiringsection 32, and a controller 33.

The signal producing section 31 is configured so as to produce a firstcontrol signal S_(i1), a second control signal S_(i2), a third controlsignal S_(i3), and a fourth control signal S_(i4). The first controlsignal S_(i1) causes the first light emitter 11 to emit the first lightbeam at a first intensity I_(o1). The second control signal S_(i2)causes the second light emitter 12 to emit the second light beam at asecond intensity I₀₂. The third control signal S_(i3) causes the thirdlight emitter 13 to emit the third light beam at a third intensity I₀₃.The fourth control signal S_(i4) causes the fourth emitter 14 to emitthe fourth light beam at a fourth intensity I₀₄. The signal producingsection 31 includes a circuit which produces the first control signalS_(i1), the second control signal S_(i2), the third control signalS_(i3), and the fourth control signal S_(i4) in accordance withinstructions from the controller 33, and an interface for outputtingthese signals.

The signal acquiring section 32 is configured so as to acquire the firstintensity signal S_(o1), the second intensity signal S_(o2), the thirdintensity signal S_(o3), and the fourth intensity signal S_(o4) whichare output from the light detector 20. The signal acquiring section 32includes an interface which receives inputs of the first intensitysignal S_(o1), the second intensity signal S_(o2), the third intensitysignal S_(o3), and the fourth intensity signal S_(o4), and a signaltransmission circuit which transmits the signals to the controller 33.

The controller 33 is communicable with the signal producing section 31and the signal acquiring section 32. The controller 33 can include oneor more processor 331 and one or more memory 332.

Examples of the processor 331 include a CPU and an MPU. The memory 332is configured so as to store instructions which may be executed by theprocessor 331. Examples of the memory 332 include a ROM which storesvarious instructions, and a RAM having a work area in which variousinstructions to be executed by the processor 331 are stored.

When instructions stored in the memory 332 are executed by the processor331, the medical photometer 30 executes the following process.

The controller 33 causes the signal producing section 31 to produce thefirst control signal S_(i1). Therefore, the first light emitter 11 emitsthe first light beam having the first wavelength λ₁, at the firstintensity I_(o1). The light detector 20 receives the first light beamwhich is transmitted through or reflected from the tissue of thesubject, and outputs the first intensity signal S_(o1) corresponding tothe intensity I₁ of the first light beam. The controller 33 causes thesignal acquiring section 32 to acquire the first intensity signalS_(o1).

A light attenuation (first light attenuation) A₁ of the first light beamby transmission through or reflection from the tissue is acquired basedon the first intensity signal S_(o1). The first light attenuation A₁ isexpressed by the following expression. Namely, the first lightattenuation A₁ may be acquired based on the first control signal S_(i1)and the first intensity signal S_(o1).

A ₁=ln(I ₀₁ /I ₁)  (1)

The controller 33 causes the signal producing section 31 to produce thesecond control signal S_(i2). Therefore, the second light emitter 12emits the second light beam having the second wavelength λ₂, at thesecond intensity I₀₂. The light detector 20 receives the second lightbeam which is transmitted through or reflected from the tissue of thesubject, and outputs the second intensity signal S_(o2) corresponding tothe intensity I₂ of the second light beam. The controller 33 causes thesignal acquiring section 32 to acquire the second intensity signalS_(o2).

A light attenuation (second light attenuation) A₂ of the second lightbeam due to transmission through or reflection from the tissue isacquired based on the second intensity signal S_(o2). The second lightattenuation A₂ is expressed by the following expression. Namely, thesecond light attenuation A₂ may be acquired based on the second controlsignal S_(i2) and the second intensity signal S_(o2).

A ₂=ln(I ₀₂ /I ₂)  (2)

The first light attenuation A₁ and the second light attenuation A₂ maybe expressed by the following expressions, respectively:

A ₁=(E _(a1) ·Hb·D _(a) +E _(v1) ·Hb·D _(v)+Σ_(t1) ·D _(t))  (3)

A ₂=(E _(a2) ·Hb·D _(a) +E _(v2) ·Hb·D _(v)+Σ_(t2) ·D _(t))  (4)

In the above expressions, E_(a) indicates the extinction coefficient (dlg⁻¹cm⁻¹) of the arterial blood, E_(v) indicates the extinctioncoefficient (dl g⁻¹cm⁻¹) of the venous blood, and Σ_(t) indicates theabsorption coefficient (cm⁻¹) due to tissue other than the blood. Thesuffix “1” indicates the first light beam, and the suffix “2” indicatesthe second light beam. Hb indicates the hemoglobin concentration inblood (g dl⁻¹), D_(a) indicates the thickness (cm) of the arterialblood, D_(v) indicates the thickness (cm) of the venous blood, and D_(t)indicates the thickness (cm) of the tissue other than the blood.

In Expressions (3) and (4), namely, the first term of the right sidecorresponds to the extent of contribution of the arterial blood of thesubject to the light attenuation, the second term of the right sidecorresponds to the extent of contribution of the venous blood of thesubject to the light attenuation, and the third term of the right sidecorresponds to the extent of contribution of the tissue of the subjectother than the blood to the light attenuation.

FIG. 2 illustrates the wavelength dependency of the extinctioncoefficient of the blood in a state where the oxygen saturation is 100%.In the embodiment, the first wavelength λ₁ (660 nm) and the secondwavelength λ₂ (700 nm) are selected as two wavelengths at which theextinction coefficients of the blood in a state where the oxygensaturation is 100% have first values that are substantially equal toeach other.

Under conditions that the oxygen saturation of the blood of the subjectis 100%, namely, E_(at) and E_(a2) in Expressions (3) and (4) may bedeemed to be approximately equal to each other. When also Σ_(t1) andΣ_(t2) are deemed to be approximately equal to each other, the followingexpression is obtained:

$\begin{matrix}\begin{matrix}{{A_{1} - A_{2}} \approx {{E_{v1} \cdot {Hb} \cdot D_{v}} - {E_{v2} \cdot {Hb} \cdot D_{v}}}} \\{= {( {E_{v1} - E_{v2}} ){{Hb} \cdot D_{v}}}}\end{matrix} & (5)\end{matrix}$

From Expressions (1) and (2), the left side of Expression (5) may berewritten as follows:

ln(I ₂ /I ₁)=(E _(v1) −E _(v2))Hb·D _(v)  (6)

In Expression (6), the first term of the left side corresponds to theratio of the first intensity signal S_(o1) and second intensity signalS_(o2) which are acquired by the signal acquiring section 32. The valuesof Ion and I₀₂ are already known, and therefore the second term of theleft side is constant. When first and second light beams are selected astwo beams for which the extinction coefficients of the blood in a statewhere the oxygen saturation is 100% are substantially equal to eachother, namely, it is possible to offset influences on extinction due tothe arterial blood and the tissue relating to the first and second lightbeams.

The controller 33 causes the signal producing section 31 to produce thethird control signal S_(i3). Therefore, the third light emitter 13 emitsthe third light beam having the third wavelength λ₃, at the thirdintensity I₀₃. The light detector 20 receives the third light beam whichis transmitted through or reflected from the tissue of the subject, andoutputs the third intensity signal S_(o3) corresponding to the intensityI₃ of the third light beam. The controller 33 causes the signalacquiring section 32 to acquire the third intensity signal S_(o3).

A light attenuation (third light attenuation) A₃ of the third light beamdue to transmission through or reflection from the tissue is acquiredbased on the third intensity signal S_(o3). The third light attenuationA₃ is expressed by the following expression. Namely, the third lightattenuation A₃ may be acquired based on the third control signal S_(i3)and the third intensity signal S_(o3).

A ₃=ln(I _(o3) /I ₃)  (7)

The controller 33 causes the signal producing section 31 to produce thefourth control signal S_(i4). Therefore, the fourth light emitter 14emits the fourth light beam having the fourth wavelength λ₄, at thefourth intensity I₀₄. The light detector 20 receives the fourth lightbeam which is transmitted through or reflected from the tissue of thesubject, and outputs the fourth intensity signal S_(o4) corresponding tothe intensity I₄ of the fourth light beam. The controller 33 causes thesignal acquiring section 32 to acquire the fourth intensity signalS_(o4).

A light attenuation (fourth light attenuation) A₄ of the fourth lightbeam due to transmission through or reflection from the tissue isacquired based on the fourth intensity signal S_(o4). The fourth lightattenuation A₄ is expressed by the following expression. Namely, thefourth light attenuation A₄ may be acquired based on the fourth controlsignal S_(i4) and the forth intensity signal S_(o4).

A ₄=ln(I ₀₄ /I ₄)  (8)

The third light attenuation A₃ and the fourth light attenuation A₄ maybe expressed by the following expressions, respectively:

A ₃=(E _(a3) ·Hb·D _(a) +E _(v3) ·Hb·D _(v)+Σ_(t3) ·D _(t))  (9)

A ₄=(E _(a4) ·Hb·D _(a) +E _(v4) ·Hb·D _(v)+Σ_(t4) ·D _(t))  (10)

In the above expressions, E_(a) indicates the extinction coefficient (dlg⁻¹cm⁻¹) of the arterial blood, E_(v) indicates the extinctioncoefficient (dl g⁻¹cm⁻¹) of the venous blood, and Σ_(t) indicates theabsorption coefficient (cm⁻¹) due to tissue other than the blood. Thesuffix “3” indicates the third light beam, and the suffix “4” indicatesthe fourth light beam. Hb indicates the hemoglobin concentration inblood (g dl⁻¹), D_(a) indicates the thickness (cm) of the arterialblood, D_(v) indicates the thickness (cm) of the venous blood, and D_(t)indicates the thickness (cm) of the tissue other than the blood.

In Expressions (9) and (10), namely, the first term of the right sidecorresponds to the extent of contribution of the arterial blood of thesubject to the light attenuation, the second term of the right sidecorresponds to the extent of contribution of the venous blood of thesubject to the light attenuation, and the third term of the right sidecorresponds to the extent of contribution of the tissue of the subjectother than the blood to the light attenuation.

As illustrated in FIG. 2 , the third wavelength λ₃ (645 nm) and thefourth wavelength λ₄ (730 nm) are selected as two wavelengths at whichthe extinction coefficients of the blood in a state where the oxygensaturation is 100% have second values that are substantially equal toeach other. The second values are different from the above-describedfirst values.

Under the conditions that the oxygen saturation of the blood of thesubject is 100%, namely, E_(a3) and E_(a4) in Expressions (9) and (10)may be deemed to be approximately equal to each other. When also E_(t)aand E_(t4) are deemed to be approximately equal to each other, thefollowing expression is obtained:

$\begin{matrix}\begin{matrix}{{A_{3} - A_{4}} \approx {{E_{v3} \cdot {Hb} \cdot D_{v}} - {E_{v4} \cdot {Hb} \cdot D_{v}}}} \\{= {( {E_{v3} - E_{v4}} ){{Hb} \cdot D_{v}}}}\end{matrix} & (11)\end{matrix}$

From Expressions (7) and (8), the left side of Expression (11) may berewritten as follows:

ln(I ₄ /I ₃)−ln(I ₀₄ /I ₀₃)=(E _(v3) −E _(v4))Hb·D _(v)  (12)

In Expression (12), the first term of the left side corresponds to theratio of the third intensity signal S_(o3) and fourth intensity signalS_(o4) which are acquired by the signal acquiring section 32. The valuesof I₀₃ and I₀₄ are already known, and therefore the second term of theleft side is constant. When first and second light beams are selected astwo beams for which the extinction coefficients of the blood in a statewhere the oxygen saturation is 100% are substantially equal to eachother, namely, it is possible to offset influences on extinction due tothe arterial blood and the tissue relating to the third and fourth lightbeams.

From Expressions (6) and (12), moreover, the following expression isobtained:

[ln(I ₄ /I ₃)−ln(I ₀₄ /I ₀₃)]/[ln(I ₂ /I ₁)−ln(I ₀₂ /I ₀₁)]=(E _(v3) −E_(v4))/(E _(v1) −E _(v2))   (13)

When two sets of two light beams for which the extinction coefficientsof the blood in a state where the oxygen saturation is 100% aresubstantially equal to each other are used, namely, it is possible tooffset influences on extinction due to at least one of the thicknessvariation of the venous blood and the variation of the hemoglobinconcentration in blood. The venous oxygen saturation of the subject maybe expressed as the function of the right side of Expression (13).

When instructions stored in the memory 332 are executed by the processor331, the medical photometer 30 calculates the venous oxygen saturationin a state where the arterial oxygen saturation of the subject is 100%,based on the first control signal S_(i1), the second control signalS_(i2), the third control signal S_(i3), the fourth control signalS_(i4), the first intensity signal S_(o1), the second intensity signalS_(o2), the third intensity signal S_(o3), and the fourth intensitysignal S_(o4).

According to the configuration, even when the arterial oxygen saturationof a subject is 100%, it is possible to determine whether the amount ofadministration of oxygen to the subject is adequate, based on thecalculated venous oxygen saturation. In other words, a novel index isprovided which, when the arterial oxygen saturation of a subject is100%, enables to determine whether the amount of administration ofoxygen to the subject is adequate.

Although not illustrated, the medical photometer 30 includes a notifyingsection. The notifying section notifies the user of the calculatedvenous oxygen saturation by using at least one of visual notification,audible notification, and haptic notification. The notifying section maybe configured so as to perform the notification only in the case wherethe calculated venous oxygen saturation suggests excessiveadministration of oxygen to the subject.

As illustrated by the broken line in FIG. 1 , the medical photometer 30may be connected to an oxygen administration apparatus 40 with wire orwireless connection. The oxygen administration apparatus 40 is anapparatus for administrating oxygen to the subject. In this case, themedical photometer 30 may cause the signal producing section 31 toproduce a control signal S which controls the operation of the oxygenadministration apparatus 40, based on the calculated venous oxygensaturation.

In the case where the calculated venous oxygen saturation suggestsinsufficient administration of oxygen, for example, the signal producingsection 31 is caused to produce a control signal which controls theoxygen administration apparatus 40 so as to increase the amount ofadministration of oxygen. In the case where the calculated venous oxygensaturation suggests excessive administration of oxygen, the signalproducing section 31 is caused to produce a control signal whichcontrols the oxygen administration apparatus 40 so as to decrease theamount of administration of oxygen.

According to the configuration, adequate administration of oxygen undera situation where the arterial oxygen saturation of a subject is 100%can be automated.

JP-A-2014-147473 discloses a technique in which an influence onextinction due to the tissue of the subject in calculation of thearterial oxygen saturation is eliminated by pressurizing the tissue toevacuate the blood. Specifically, the tissue of the subject isirradiated by first and second light beams of different wavelengths. Ina first state where the tissue is not pressurized, received lightintensities of the first and second light beams are acquired. In asecond state where the tissue is pressurized, then, received lightintensities of the first and second light beams are acquired. A firstlight attenuation is acquired from the received light intensity of thefirst light beam which is acquired in the first state, and that of thefirst light beam which is acquired in the second state. A second lightattenuation is acquired from the received light intensity of the secondlight beam which is acquired in the first state, and that of the secondlight beam which is acquired in the second state. The arterial oxygensaturation of the subject is calculated based on the difference betweenthe first light attenuation and the second light attenuation.

Referring to the technique, Expressions (1), (2), (7), and (8) may berewritten respectively as follows:

A ₁=ln(I ₂₁ /I ₁₁)  (14)

A ₂=ln(I ₂₂ /I ₁₂)  (15)

A ₃=ln(I ₂₃ /I ₁₃)  (16)

A ₄=ln(I ₂₄ /I ₁₄)  (17)

In the above, I₁₁, I₁₂, I₁₃, and I₁₄ indicate the intensities of thefirst, second, third, and fourth light beams which are acquired by thelight detector 20 in a first state, respectively, and I₂₁, I₂₂, I₂₃, andI₂₄ indicate the intensities of the first, second, third, and fourthlight beams which are acquired by the light detector 20 in a secondstate, respectively. The first state corresponds to a state where thetissue of the subject is not pressurized. The second state correspondsto a state where the tissue of the subject is pressurized.

Expression (18) is obtained based on the description referring toExpressions (3) to (6). Similarly, Expression (19) is obtained based onthe description referring to Expressions (9) to (12).

ln(I ₁₂ /I ₁₁)−ln(I ₂₂ /I ₂₁)=(E _(v1) −E _(v2))Hb·D _(v)  (18)

ln(I ₁₄ /I ₁₃)−ln(I ₂₄ /I ₂₃)=(E _(v3) −E _(v4))Hb·D _(v)  (19)

Then, the following expression is obtained based on the descriptionreferring to Expression (13):

[ln(I ₁₄ /I ₁₃)−ln(I ₂₄ /I ₂₃)]/[ln(I ₁₂ /I ₁₁)−ln(I ₂₂ /I ₂₁)]=(E _(v3)−E _(v4))/(E _(v1) −E _(v2))   (20)

The intensity I₁₁ of the first light beam, intensity I₁₂ of the secondlight beam, intensity I₁₃ of the third light beam, and intensity I₁₄ ofthe fourth light beam which are acquired by the light detector 20 in thefirst state correspond to the first intensity signal S_(o1), the secondintensity signal S_(o2), the third intensity signal S_(o3), and thefourth intensity signal S_(o4) which are acquired from the lightdetector 20 in the first state, respectively. The intensity I₂₁ of thefirst light beam, intensity I₂₂ of the second light beam, intensity 123of the third light beam, and intensity I₂₄ of the fourth light beamwhich are acquired by the light detector 20 in the second statecorrespond to the first intensity signal S_(o1), the second intensitysignal S_(o2), the third intensity signal S_(o3), and the fourthintensity signal S_(o4) which are acquired from the light detector 20 inthe second state, respectively.

Therefore, the venous oxygen saturation in a state where the arterialoxygen saturation of the subject is 100% may be calculated based on: afirst signal set including the first intensity signal S_(o1), the secondintensity signal S_(o2), the third intensity signal S_(o3), and thefourth intensity signal S_(o4) which are acquired in the first state;and a second signal set including the first intensity signal S_(o1), thesecond intensity signal S_(o2), the third intensity signal S_(o3), andthe fourth intensity signal S_(o4) which are acquired in the secondstate. FIG. 3 illustrates an example of the operation of the medicalphotometer 30 which is based on the technique.

In the state where the tissue is not pressurized, firstly, the firstsignal set including the first intensity signal S_(o1), the secondintensity signal S_(o2), the third intensity signal S_(o3), and thefourth intensity signal S_(o4) is acquired (STEP 1).

Then, the tissue is pressurized (STEP 2). The pressurization may beperformed by the hand of the user, or by a cuff.

In the case where the pressurization is performed by the hand of theuser, it is preferable to notify the user of the timing of thepressurization by using at least one of visual notification, audiblenotification, and haptic notification. The notification may function asa trigger for the medical photometer 30 to acquire the second signalset. Alternatively, the user may input the timing of pressurization tothe medical photometer 30 by operating a button or the like.

In the case where the pressurization is performed by a cuff, the medicalphotometer 30 may include a pressure controller which controls theinternal pressure of the cuff. The pressure controller may include apump. The operation of the pressure controller may be controlled by thecontroller 33. The pressurization by the cuff may be performed inresponse to, for example, a button operation conducted by the user, orautomatically by the controller 33.

In the state where the tissue is pressurized, the second signal setincluding the first intensity signal S_(o1), the second intensity signalS_(o2), the third intensity signal S_(o3), and the fourth intensitysignal S_(o4) is acquired (STEP 3).

Then, the venous oxygen saturation in a state where the arterial oxygensaturation of the subject is 100% is calculated based on the acquiredfirst and second signal sets (STEP 4).

According to the configuration, simply by changing the pressurizationstate of the tissue, the venous oxygen saturation in a state where thearterial oxygen saturation of the subject is 100% may be calculatedbased on only the first intensity signal S_(o1), the second intensitysignal S_(o2), the third intensity signal S_(o3), and the fourthintensity signal S_(o4) which are acquired from the light detector 20.Even when the arterial oxygen saturation of a subject is 100%, namely,it is possible to determine whether the amount of administration ofoxygen to the subject is adequate, based on the calculated venous oxygensaturation.

In the example, the state where the tissue is not pressurizedcorresponds to the first signal set, and the state where the tissue ispressurized corresponds to the second signal set. Alternatively, thestate where the tissue is pressurized may correspond to the first signalset, and the state where the tissue is not pressurized may correspond tothe second signal set.

The above-described embodiment is a mere example for facilitatingunderstanding of the presently disclosed subject matter. Theconfiguration of the embodiment may be adequately changed or improvedwithout departing from the spirit of the presently disclosed subjectmatter. It is obvious that equivalents are included within the technicalscope of the presently disclosed subject matter.

In the above-described embodiment, 660 nm and 700 nm are selected as theset of the first wavelength λ₁ and the second wavelength λ₂, and 645 nmand 730 nm are selected as the set of the third wavelength λ₃ and thefourth wavelength λ₄. However, at least one of the two sets may bechanged to arbitrary two wavelengths at which the extinctioncoefficients of the blood in a state where the oxygen saturation is 100%are substantially equal to each other.

Only the configuration for calculating the venous oxygen saturation of asubject is disposed in the above-described embodiment. However, aconfiguration for calculating the arterial oxygen saturation of thesubject may be disposed. In this case, at least one of components (lightemitters, a light detector, a signal producing section, a signalacquiring section, a controller, and the like) may be shared with acorresponding component in the embodiment and for calculating the venousoxygen saturation. The value of the calculated arterial oxygensaturation may be used for determining insufficient administration ofoxygen to the subject.

What is claimed is:
 1. A medical photometer comprising: a signalacquiring section configured to acquire: a first intensity signalcorresponding to an intensity of a first light beam which is transmittedthrough or reflected from tissue of a subject, and which has a firstwavelength; a second intensity signal corresponding to an intensity of asecond light beam which is transmitted through or reflected from thetissue, and which has a second wavelength; a third intensity signalcorresponding to an intensity of a third light beam which is transmittedthrough or reflected from the tissue, and which has a third wavelength;and a fourth intensity signal corresponding to an intensity of a fourthlight beam which is transmitted through or reflected from the tissue,and which has a fourth wavelength; a processor; and a memory that storesinstructions executable by the processor, wherein the first wavelengthand the second wavelength are selected as two wavelengths at each ofwhich an extinction coefficient of blood in a state where an oxygensaturation is 100% has a first value, the third wavelength and thefourth wavelength are selected as two wavelengths at each of which theextinction coefficient of the blood in a state where the oxygensaturation is 100% has a second value that is different from the firstvalue, and, when the instructions are executed by the processor, themedical photometer acquires a first signal set including the firstintensity signal, second intensity signal, third intensity signal, andfourth intensity signal in a state where the tissue is not pressurized,acquires a second signal set including the first intensity signal,second intensity signal, third intensity signal, and fourth intensitysignal in a state where the tissue is pressurized, and calculates avenous oxygen saturation in a state where an arterial oxygen saturationof the subject is 100%, based on the first signal set and the secondsignal set.
 2. The medical photometer according to claim 1, wherein thefirst wavelength is 660 nm, the second wavelength is 700 nm, the thirdwavelength is 645 nm, and the fourth wavelength is 730 nm.
 3. Themedical photometer according to claim 1, further comprising a signalproducing section that produces a control signal which controls anoperation of an oxygen administration apparatus for administratingoxygen to the subject, and, when the instructions are executed by theprocessor, the signal producing section is caused to produce the controlsignal, based on the calculated venous oxygen saturation.
 4. A medicalphotometry system comprising: a first light emitter that emits a firstlight beam having a first wavelength; a second light emitter that emitsa second light beam having a second wavelength; a third light emitterthat emits a third light beam having a third wavelength; a fourth lightemitter that emits a fourth light beam having a fourth wavelength; alight detector that outputs a first intensity signal corresponding to anintensity of the first light beam that is transmitted through orreflected from tissue of a subject, a second intensity signalcorresponding to an intensity of the second light beam that istransmitted through or reflected from the tissue, a third intensitysignal corresponding to an intensity of the third light beam that istransmitted through or reflected from the tissue, and a fourth intensitysignal corresponding to an intensity of the fourth light beam that istransmitted through or reflected from the tissue; and a medicalphotometer to which the first light emitter, the second light emitter,the third light emitter, the fourth light emitter, and the lightdetector are connected with wire or wireless connection, wherein themedical photometer includes: a signal acquiring section configure toacquire the first intensity signal, the second intensity signal, thethird intensity signal, and the fourth intensity signal; a processor;and a memory that stores instructions executable by the processor, thefirst wavelength and the second wavelength are selected as twowavelengths at each of which an extinction coefficient of blood in astate where an oxygen saturation is 100% has a first value, the thirdwavelength and the fourth wavelength are selected as two wavelengths ateach of which the extinction coefficient of the blood in a state wherethe oxygen saturation is 100% has a second value that is different fromthe first value, and, when the instructions are executed by theprocessor, the medical photometer acquires a first signal set includingthe first intensity signal, second intensity signal, third intensitysignal, and fourth intensity signal in a state where the tissue is notpressurized, acquires a second signal set including the first intensitysignal, second intensity signal, third intensity signal, and fourthintensity signal in a state where the tissue is pressurized, andcalculates a venous oxygen saturation in a state where an arterialoxygen saturation of the subject is 100%, based on the first signal setand the second signal set.